Thanks Eik...
..the uniroot() was the function I was looking for.

Best regards,
/J


On Thu, Dec 12, 2013 at 4:27 PM, Eik Vettorazzi <E.Vettorazzi@uke.de> wrote:

> Assuming r, sd1 and sd2 as known(fixed) components, this works:
>
> X <- pnorm(q=q,sd=sd1)*r+pnorm(q=q,sd=sd2)*(1-r)
>
> uniroot(function(x)pnorm(q=x,sd=sd1)*r+pnorm(q=x,sd=sd2)*(1-r)-X,c(-1e6,1e6))
>
>
> Cheers
>
> Am 12.12.2013 15:58, schrieb Johannes Radinger:
> > Thanks for the fast response. Of course I totally overlooked qnorm as I
> had
> > a more complex task in my head.
> >
> > I wanted to reverse following equation:
> > r=0.67
> > q=-150
> > sd1=100
> > sd2=1000
> >
> > X <- pnorm(q=q,sd=sd1)*r+pnorm(q=q,sd=sd2)*(1-r)
> >
> > Maybe its mathematically really easy, but somehow I don't get it how to
> do
> > reverse and provide X and
> > get q especially with the presence of r respectively as weighting factors
> > for the two distributions.
> >
> > /J
> >
> >
> > On Thu, Dec 12, 2013 at 3:09 PM, Dániel Kehl <kehld@ktk.pte.hu> wrote:
> >
> >> Looks like homework.
> >>
> >> Try ?qnorm
> >> ________________________________________
> >> Feladó: r-help-bounces@r-project.org [r-help-bounces@r-project.org] ;
> >> meghatalmaz&#243;: Johannes Radinger [johannesradinger@gmail.com]
> >> Küldve: 2013. december 12. 14:56
> >> To: R help
> >> Tárgy: [R] Solving a normal distribution pnorm for q
> >>
> >> Hi,
> >>
> >>
> >>
> >> I found follwowing example of pnorm here:
> >>
> >>
> >>
> http://www.r-tutor.com/elementary-statistics/probability-distributions/normal-distribution
> >>
> >>
> >>
> >> Problem
> >>
> >> Assume that the test scores of a college entrance exam fits a normal
> >> distribution. Furthermore, the mean test score is 72, and the standard
> >> deviation is 15.2. What is the percentage of students scoring 84 or
> more in
> >> the exam?
> >>
> >>
> >>
> >> Solution
> >>
> >>> pnorm(84, mean=72, sd=15.2, lower.tail=FALSE)
> >>
> >> [1] 0.21492
> >>
> >>
> >>
> >> That is straight forward, however what if I want to know the score the
> best
> >> 30% students are reaching at least. So I know the solution of pnorm but
> >> want to know its q. How can that be achieved in R?
> >>
> >>
> >>
> >> Any suggestions?
> >>
> >>
> >>
> >> /Johannes
> >>
> >>         [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help@r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> >> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
> >       [[alternative HTML version deleted]]
> >
> >
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>
> --
> Eik Vettorazzi
> Institut für Medizinische Biometrie und Epidemiologie
> Universitätsklinikum Hamburg-Eppendorf
>
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> 20246 Hamburg
>
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